Adhesives have been utilized for decades to bind a wide variety of materials together, and new applications for adhesives are being developed on a continuing basis. Compared to mechanical fastening devices, adhesives are capable of distributing stresses over wider surface areas and provide superior strength-to-weight ratios and dimensional stability for a variety of finished products.
A large portion of the adhesives utilized in industrial and commercial applications contain organic solvents and/or nonaqueous diluents which function to reduce the viscosity of the adhesives to facilitate their application. A variety of application methods are employed in industry, such as brushing, rolling, dipping and the like, but spraying of adhesive in atomized droplets is often the most desirable particularly when adhering materials such as paper, cardboard, wood, leather, plastic, cloth and rubber. A growing concern with the application of adhesives containing organic solvents or nonaqueous diluents, particularly when employing the spraying method of application, is the environmental hazards occasioned by the release of organic solvent vapors to atmosphere during the application procedure. The escaping solvents can contaminate the surrounding atmosphere and create injury to the personnel involved in the application operation. Additionally, since most solvents react with oxidants, pollution problems of toxicity, odor and smog may be created.
While it may be possible to simply reduce the organic solvent and/or nonaqueous diluent content of certain adhesives, such a reduction generally increases the viscosity of the resulting adhesive composition to such an extent that it can no longer be sprayed using conventional techniques, i.e. air spray, airless spray and air-assisted, airless spray. Even if such highly viscous adhesive compositions are capable of being sprayed, the pressure required to convey and transmit the adhesive through an adhesive dispenser to obtain successful atomization is very high. In some cases, even at high application pressures, the adhesive layer resulting from spraying viscous adhesive compositions does not have enough solvent present to allow for sufficient flow out of the adhesive where a continuous, uniform adhesive coating is desired.
These problems have been addressed in U.S. Pat. No. 5,066,522 to Cole, et al., in which it is suggested that at least a portion of the organic solvents and/or nonaqueous diluents in conventional adhesive compositions be replaced with a supercritical fluid as a diluent. As discussed in U.S. Pat. No. 5,066,522, supercritical carbon dioxide has been found to be an acceptable replacement for a substantial portion of the organic solvents or nonaqueous diluents currently utilized in many adhesive compositions. The presence of supercritical carbon dioxide in solution with the adhesive reduces the environmental hazards created by the escape of solvents, enhances curing of the adhesive for certain types of adhesive materials, and often avoids the formation of "cobwebs" or elongated, thin strands of adhesive which may be produced during spraying operations with conventional adhesive formulations. The problem of cobwebs is eliminated because of the explosive atomization created when the supercritical carbon dioxide is released from solution upon exposure of the pressurized solution to atmosphere.
A batch type system and a continuous supply system are disclosed in U.S. Pat. No. 5,066,522, and one or the other is employed depending upon the requirements of the particular application. The effectiveness of each of these systems in spraying an acceptable coating or layer of viscous adhesive onto a substrate is dependent, at least in part, on the effectiveness with which the virgin adhesive and supercritical carbon dioxide are intermixed to form an adhesive composition wherein the supercritical carbon dioxide is dissolved or dispersed in solution with the adhesive. In both the batch type and continuous supply systems disclosed in U.S. Pat. No. 5,066,522, the supercritical carbon dioxide and adhesive are combined within a static mixer located at a position downstream from the sources of both materials. It has been found that static mixers are ineffective in obtaining complete admixture of the supercritical carbon dioxide and adhesive especially when the virgin adhesive has a high solids content, i.e. on the order of at least about 20% to 30% solids.
The problem with static mixers is particularly evident when attempting to spray rubber-based cements, especially when carbon gels are present. These types of adhesives contain "bound rubbers" which means that the rubber and solvent components of the adhesive form a suspension or dispersion as opposed to a solution. Most bound rubber adhesives use heptane as a solvent to reduce their viscosity. When these types of adhesives are passed through a static mixer with supercritical carbon dioxide, the heptane preferentially dissolves into the supercritical carbon dioxide, leaving the rubber component. The result is pockets of highly solvated supercritical carbon dioxide in a thick, high viscosity rubber, which cannot be effectively sprayed.